The present invention relates to an astigmatism measuring method, an astigmatism adjusting method and an astigmatism measuring system, all for an optical pickup which applies a light beam converged by an objective lens to a recording medium.
The present application claims priority from Japanese Application No. 2003-39652, the disclosure of which is incorporated herein by reference.
For example, with an optical pickup for use in recording/reproducing information on an optical disk such as CD or DVD, it is required to correctly and constantly converge a light beam (for writing or reproducing information) on information pit train on the surface of the optical disk. Because of this, it is absolutely necessary to reduce as much as possible an aberration (specifically, an astigmatism) of the optical system of the optical pickup.
In order to measure an astigmatism of an optical pickup, it is usual to employ a method using a laser interferometer. However, this method needs considerable working time and requires an experienced labor for correctly making coincidence between the optical axis of the laser interferometer and that of the optical pickup, thus rendering it unsuitable for adjusting the optical system of an optical pickup in an actual manufacturing process.
In order to solve the above problem, Japanese Unexamined Patent Application Publication No. 2002-15435 has suggested an improved method for adjusting an astigmatism of an optical pickup, which can be used with an easy manner in an actual manufacturing process. As shown in FIG. 11 of the published patent application, the suggested adjustment method requires that a spot image (light beam sectional shape along a direction orthogonal to the advancing direction of the light beam) of a light beam converged near the focus of an optical pickup be taken as image data into a processing system, thereby image-processing and measuring a roundness (aspect ratio) of the spot image. Then, an incident angle of the light beam entering the objective lens is changed such that the roundness of the spot image becomes maximum (almost equal to 1), thereby correcting the astigmatism of an optical pickup.
However, the above-mentioned astigmatism adjusting method requires a precondition that the incident angle of a light beam is changed to finally obtain a spot image which is a true circle (or a generally true circle). Namely, since the above-mentioned adjusting method takes a spot image of true circle as an adjustment standard, it was found difficult to minimize an astigmatism by such an astigmatism adjusting method (if the objective lens of an optical pickup has an inherent aberration and hence a converged spot image fails to become a true circle even though the incident angle of the light beam is adjusted). In other words, if the above-mentioned astigmatism adjusting method is to be used for performing an optimum astigmatism adjustment, it is necessary to use high quality optical elements (having less astigmatism) to construct the optical system of an optical pickup with a high precision, so as to render the spot image to become a true circle.